RIBOSOMAL GENE PROMOTER DOMAINS CAN FUNCTION AS ARTIFICIAL ENHANCERS OF RNA-POLYMERASE-I TRANSCRIPTION, SUPPORTING A PROMOTER ORIGIN FOR NATURAL ENHANCERS IN XENOPUS

Enhancers of RNA polymerase I transcription in higher eukaryotes are repetitive elements within the intergenic spacers of rRNA genes. In Xenopus and mouse, enhancers and the gene promoter bind the activator protein, upstream binding factor, and in Xenopus, enhancers also share sequence similarity with an upstream domain of the promoter. This upstream promoter domain can act as an efficient enhancer when polymerized and cloned adjacent to a ribosomal gene promoter injected into oocytes. A core promoter domain lacking similarity with spacer sequences in Xenopus laevis but analogous to a repeated sequence in Xenopus borealis can also function as an enhancer. These data demonstrate functional relatedness between the promoter and enhancers, supporting the hypothesis that enhancers could have evolved from duplicated promoter domains that bind essential transcription factors. The ability of upstream binding factor to bind enhancers inactivated by mutation suggests that upstream binding factor binding alone cannot explain enhancer function.